In various applications, especially military, thee are vehicles, particularly air vehicles, that are provided with sensors for detecting radiation such as incoming IR-rays, visible light or radar rays, usually in a direction directly opposite to the direction of travel of the vehicle. There is a need for protection devices intended to protect the sensor means up to the moment when it is activated. At that moment the vehicle is usually traveling at a relatively high speed through its medium, air or water. When the sensor means is to be activated the protection device must be removed almost instantaneously in order to permit the sensor means to receive radiation. In view of said relatively high speed through a medium, the speed will exert a force directed opposite to the direction of travel of the vehicle, i.e. it will press the protection device towards the vehicle. At the removal of the protection device said force must be overcome and the protection device be taken from the direction of travel of the vehicle, i.e. away from the active direction of the sensor means, which is usually an axis of symmetry through the sensor means. In addition to meeting these said requirements the protection device must be absolutely reliable in the prevailing operating conditions, i.e. a wide temperature range, exposed to vibrations etc., and must be reasonably inexpensive to manufacture.